Because of the severe shortage of available human organs for donation, other species have been considered as potential donors for human transplantation. Of the options available, the pig appears to be the most suitable candidate. Hyperacute rejection has been a major barrier to pig- to-human transplantation, although several strategies have been successful in preventing this. Despite preventing hyperacute rejection, delayed xenografic rejection occurs which is characterized by endothelial cell activation and invasion of macrophages and natural killer cells. Although many of the successful strategies to avert hyperacute rejection have used approaches which prevent complete activation of the complement cascade, none have attempted to inhibit the complement cascade in its earliest phases. Deposition of the initial components of the complement cascade (C1, C4, C2) can lead to the accumulation of inflammatory cells and may contribute to endothelial cell activation thus creating a situation which is consistent with delayed xenograft rejection. Using novel synthetic peptides (complementary binding peptides) developed at Northwestern University, we will attempt to block the deposition of the early components of the complement cascade, thus preventing hyperacute rejection as well as the pro-inflammatory stimulus which is elicited from these early components. This will be tested in an ex vivo perfusion circuit where human blood is perfused through a pig heart with and without the complementary binding peptides.